Generally, licenses of frequencies are managed by the government and only licensed persons can use the frequencies in specific places and time under strict management. Demands for the wireless frequencies as finite resource are increasing and a lack of allocatable frequencies is problematic around the world.
Therefore, in recent years, a method has been studied for using a frequency band (white space) which is not used spatially and temporally irrespective of being already assigned as a using method of new frequency in order to solve the problem of exhausted frequencies. A cognitive wireless technique is used such that while effects on the use of frequencies in existing systems of the licensed users (which will be denoted as “primary users” below) are sufficiently avoided, non-licensed users (which will be denoted as “secondary users” below) flexibly use a white space.
In a method for correctly recognizing which frequency channel is in a white space, a database (DB) server for managing a list of white space channels (WSCH) and providing positional information, antenna height or antenna directivity, gain and the like is installed over the Internet such that each wireless station can access the same directly or via a proxy server. Each wireless station acquires its own WSCH list (list of available frequency channels), maximum transmittable power corresponding to each WSCH, available period, and the like from the DB server.
In other method, each wireless station senses a radio wave used by a primary system with spectrum sensing, and when confirming the absence of a radio wave, enables a frequency channel of the primary system to be used as a white space, and when sensing the presence thereof, excludes the channel from its WSCH list.
There is known IEEE 802.22 (see Non-Patent Literature 2) as an international standard-setting organization of wireless communication systems using a white space. FIG. 1 illustrates a system structure of the IEEE 802.22-2011 (simply called 802.22 below). The system is such that one base station (BS) and one or more pieces of customer premises equipment (CPE) configure a cell, and avoids interference on the primary system by an access to a DB server 6 via the Internet 5 thereby to realize secondary use.
The management or setting of a channel operated by the 802.22 system is controlled by a spectrum manager (SM) in the base station, and the SM sets priorities on the WSCH to select one operating channel for use based on a DB access result (WSCH list for BS) acquired by a management information base (MIB), a spectrum sensing result and positional information.
Exemplary operations of the 802.22 system will be described below with reference to FIG. 2.
When being powered on and activated, a BS acquires a WSCH list by an access to the DB server 6 and spectrum sensing, and selects one channel as operating channel from the WSCH list thereby to start operation. That is, a wireless signal is exchanged at a frequency of the operating channel.
When starting operation, the BS broadcasts control information to a service area (cell). The 802.22 employs a structure in which 16 frames are assumed as one superframe as illustrated in FIG. 3, and the BS periodically transmits SCH (Superframe Control Header) as superframe control information, FCH (Frame Control Header) as frame control information, DS-MAP (Downstream Map) or US-MAP (Upstream Map) to manage and control the cell.
When being powered on, CPE confirms a channel at which a primary system is absent by sensing, and then tries to receive a BS signal (SCH) while switching frequencies in a BS search processing. (Sensing may be included in the BS search processing or sensing may be performed on the channel after a BS signal (SCH) is sensed.)
When successfully receiving SCH from the BS, and additionally receiving the frame control information such as FCH, DS-MAP or US-MAP, the CPE can correctly recognize the structure in the frame thereby to perform a synchronization processing such as adjustment of signal transmission/reception timing or transmission power between BS and CPE, or procedures such as registration, authentication and service allocation of CPE terminal information (ID, positional information, maximum transmission power). At this time, CPE presents its positional information, thereby asking BS whether the incumbent operating CH is available as its WSCH.
When connection between the BS and the CPE is established, data communication is performed under control of the BS. The 802.22 employs OFDMA (Orthogonal Frequency Division Multiple Access) for multiple access system and TDD (Time Division Duplex) for complex communication system.
While the BS performs data communication, it updates the WSCH list by DB access or sensing periodically or as needed, and at this time, when the BS determines that the operating channel is not available, a channel switch processing is performed to switch the channels in the entire cell. When only specific CPE cannot use an incumbent operating channel, only the CPE is disconnected, and a determination to continue the operation at the channel may be made depending on a BS operation policy.
While the CPE performs data communication, it performs sensing periodically or in response to an instruction from BS, and when CPE senses a primary system at the operating channel, it notifies the information to the BS. The BS performs channel switching with the notification as a trigger. When the CPE cannot receive a signal from the BS over a certain period of time due to failed reception of a control message such as channel switch request from the BS, the CPE achieves the channel switching in the BS search processing.